How The Ancients Measured Small Electrical Resistances

[John L Stewart]

Behold, The Kelvin Bridge-

http://en.wikipedia.org/wiki/Kelvin_bridge

Cheers, John

[Patrick Turner]

On Mar 9, 7:21*am, John L Stewart <John.L.Stewart.
> wrote:
> Behold, The Kelvin Bridge-
>
> http://en.wikipedia.org/wiki/Kelvin_bridge
>
> Cheers, John
>
> --
> John L Stewart

With such an atrociously small amount of detail about the 2 ganged
pots in the above Wikipedia link, and a hopelessly inadequate
explanation for dummies, I expect nobody at r.a.t will build a Kelvin
bridge any time soon.

But don't worry, be happy, because anyone with brain will figure out
how to do what the ancients of 1912 or anyone else did, ie, to
measures low R **well enough**
using *one) fixed R and a **good** millivoltmeter, and a Vdc supply -
can be varaible to suit value of R so that Idc is not dangerously
high.
The R could also be a pack of switched R of known values, so that an
ohm meter is not required, just the VM, and Ohm's Law.

Some might use an additional pot, say 10k linear, and make a simple
bridge, then null the voltage across the bridge and calculate the low
R.
If the Rin of the VM > 9M, then it won't affect the measurements.

But of course a Vdc supply is needed, and an ohm meter which does down
to 10 ohms accurately.
Once set up, one can measure low R quickly, because you damn well
don't want to **** around all day just to measure R. With the simple
bridge an accurate VM is not needed, because you are measuring the
*absense* of a voltage, and a VM built with an opamp & NFB will sure
be good to measure 0V or very close to it between two circuit points.
But a 20,000 ohms/V analog meter might do. When the pot is adjsted to
get the null there should be no current in the VM so it does not
matter much what the input resistance of the meter is.

Patrick Turner.

[Big Bad Bob]

On 03/09/11 14:42, Patrick Turner so witilly quipped:
> But a 20,000 ohms/V analog meter might do. When the pot is adjsted to
> get the null there should be no current in the VM so it does not
> matter much what the input resistance of the meter is.

Right - no current flow at the NULL point. 'Crappy ohm/volt' analog
meters do fine for this.

As an alternate kind of 'bridge', I have measured capacitors that are
either unmarked or 'marked in a weird manner' by creating capacitor
bridges - 60hz output from a 12.6V transformer, measure the AC volts
across the known and the unknown cap. This works well with a 0.1 uF cap
in series down to about 100pf (below which you can then use smaller
series caps). In this case I measured total volts and 'across the
unknown' (so my meter's impedence didn't throw anything off).

yeah, lots of ways to skin cats / disrobe / measure electronic components.

[John L Stewart]

On Wed, 09 Mar 2011 19:09:29 -0800, Big Bad Bob
> wrote:

>On 03/09/11 14:42, Patrick Turner so witilly quipped:
>> But a 20,000 ohms/V analog meter might do. When the pot is adjsted to
>> get the null there should be no current in the VM so it does not
>> matter much what the input resistance of the meter is.
>
>Right - no current flow at the NULL point.

That's true for as far as it goes but 'at the NULL point' isn't the
issue with a bridge, it's whether the meter can well enough read NOT
'at the null point' so one may then accurately determine the null.


> 'Crappy ohm/volt' analog
>meters do fine for this.

True for the 'measuring small electrical resistances' application.

>As an alternate kind of 'bridge', I have measured capacitors that are
>either unmarked or 'marked in a weird manner' by creating capacitor
>bridges - 60hz output from a 12.6V transformer, measure the AC volts
>across the known and the unknown cap. This works well with a 0.1 uF cap
>in series down to about 100pf (below which you can then use smaller
>series caps). In this case I measured total volts and 'across the
>unknown' (so my meter's impedence didn't throw anything off).
>
>yeah, lots of ways to skin cats / disrobe / measure electronic components.
>
>

Here Ya Are........for those too busy to Google more info.

http://www.answers.com/topic/kelvin-bridge

Makes a lot of sense to me.

Information only. The ancients did not have all the goodies we have. Pls thank them for bequeathing those goodies to us!

Cheers, John

[John L Stewart]

On Thu, 10 Mar 2011 11:11:30 +0000, John L Stewart
> wrote:

>
>flipper;927688 Wrote:
>> On Wed, 09 Mar 2011 19:09:29 -0800, Big Bad Bob
>> > wrote:
>> -
>> >On 03/09/11 14:42, Patrick Turner so witilly quipped:-
>> >> But a 20,000 ohms/V analog meter might do. When the pot is adjsted
>> to
>> >> get the null there should be no current in the VM so it does not
>> >> matter much what the input resistance of the meter is.-
>> >
>> >Right - no current flow at the NULL point.-
>>
>> That's true for as far as it goes but 'at the NULL point' isn't the
>> issue with a bridge, it's whether the meter can well enough read NOT
>> 'at the null point' so one may then accurately determine the null.
>>
>> -
>> > 'Crappy ohm/volt' analog
>> >meters do fine for this.-
>>
>> True for the 'measuring small electrical resistances' application.
>> -
>> >As an alternate kind of 'bridge', I have measured capacitors that are
>> >either unmarked or 'marked in a weird manner' by creating capacitor
>> >bridges - 60hz output from a 12.6V transformer, measure the AC volts
>> >across the known and the unknown cap. This works well with a 0.1 uF
>> cap
>> >in series down to about 100pf (below which you can then use smaller
>> >series caps). In this case I measured total volts and 'across the
>> >unknown' (so my meter's impedence didn't throw anything off).
>> >
>> >yeah, lots of ways to skin cats / disrobe / measure electronic
>> components.
>> >
>> >-
>
>Here Ya Are........for those too busy to Google more info.
>
>http://www.answers.com/topic/kelvin-bridge
>
>Makes a lot of sense to me.
>
>Information only. The ancients did not have all the goodies we have. Pls
>thank them for bequeathing those goodies to us!
>
>Cheers, John

I think this one is a little better in that it gives the evolution and
explains why.

http://www.allaboutcircuits.com/vol_1/chpt_8/10.html

That is the best explanation I've seen on the Kelvin Bridge. Certainly easier to follow than what is in the text book I used 50 years ago.

Cheers to all, John